26 DHT pre-amplifier (Gen1)

My old 26 pre-amplifier found a new home recently so I was sad to see it departing after so many years of fantastic sound. I’m sure the new owner will enjoy it as much as I had.

Before dispatching it a minor upgrade was done by replacing the old SSHV1 for a newer SSHV2 and readjusting the bias point based on my experience with the Gen2 and Gen3 builds.

26 Preamp Gen 1_2The 26 is biased to 160V and 7mA. Filaments are starved down to 850mA, however 900mA should be as good if you’re not experiencing any microphonic noise subject to the 26 valves you have in your stock.

The supply circuit has been mentioned many times before, and here is the final build:

26 preamp overview

 

Pre-amplifier Performance & key measurements

  1. THD <0.02% @ 6dB
  2. Bandwidth: 10 Hz – 50kHz (-3dB)
  3. Output noise (inband): <300uV
  4. 50 Hz hum level: <-75dB (subject to valve used)

This preamp is extremely quiet, here is a measurement of the output noise:

In-band noise is lower than 300μV but this will vary depending on the noise picked up by the 26/226 valve used. I found this to be the average of a good quality 26:

noise out right

 

Frequency response:

Flat response from 10Hz up to 50kHz (-3dB):

Freq response

 

The distortion is very low up to 0.02% for 6dBV:

Level response

 

 

This preamp is a fantastic piece of gear and will surely get one of this up again and running shortly.

 

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